The IEEE 1394 standard, "P1394 Standard For A High Performance Serial Bus," Draft 8.01v1, Jun. 16, 1995, is an international standard for implementing an inexpensive high-speed serial bus architecture which supports both asynchronous and isochronous format data transfers. The IEEE 1394 standard provides a high-speed serial bus for interconnecting digital devices thereby providing a universal I/O connection. The IEEE 1394 standard defines a digital interface for the applications thereby eliminating the need for an application to convert digital data to analog data before it is transmitted across the bus. Correspondingly, a receiving application will receive digital data from the bus, not analog data, and will therefore not be required to convert analog data to digital data. An `application` as used herein will refer to either an application or a device driver.
The cable specified by the IEEE 1394 standard is very thin in size compared to many other cables, such as conventional co-axial cables, used to connect such devices. Devices can be added and removed from an IEEE 1394 bus while the bus is active. If a device is so added or removed the bus will then automatically reconfigure itself for transmitting data between the then existing nodes. A node is considered a logical entity with a unique address on the bus structure. Each node provides an identification ROM, a standardized set of control registers and its own address space.
A standard IEEE 1394 cable is illustrated in FIG. 1. An IEEE 1394 network using the standard IEEE 1394 cable 10 is a differential, copper wire network, which includes two differential pairs of wires 12 and 14, carrying the differential signals TPA and TPB, respectively. As shown in FIG. 1, the pairs of wires 12 and 14 are twisted together within the cable 10. The signals TPA and TPB are both low voltage, low current, bidirectional differential signals used to carry data bits or arbitration signals. The signals TPA and TPB have a maximum specified amplitude of 265 mVolts. The twisted pairs of wires 12 and 14 have a relatively high impedance, specified at 110 ohms, such that minimal power is needed to drive an adequate signal across the wires 12 and 14. The standard IEEE 1394 cable 10 also includes a pair of power signals VG and VP, carried on the wires 16 and 18, respectively. The wires 16 and 18 are also twisted together within the cable 10. The pair of power signals VP and VG provide the current needed by the physical layer of the serial bus to repeat signals. The wires 16 and 18 have a relatively low impedance and are specified to have a maximum power level of 60 watts.
The IEEE 1394 cable environment is a network of nodes connected by point-to-point links, including a port on each node's physical connection and the cable between them. The physical topology for the cable environment of an IEEE 1394 serial bus is a non-cyclic network of multiple ports, with finite branches. The primary restriction on the cable enviromnment is that nodes must be connected together without forming any closed loops.
The IEEE 1394 cable connects ports together on different nodes. Each port includes terminators, transceivers and simple logic. A node can have multiple ports at its physical connection. The cable and ports act as bus repeaters between the nodes to simulate a single logical bus. Because each node must continuously repeat bus signals, the separate power VP wire 18 and ground VG wire 16, within the cable 10, enable the physical layer of each node to remain operational even when the local power at the node is turned off. The pair of power wires 16 and 18 can even be used to power an entire node if it has modest power requirements. The signal VG carried on the wire 16 is a grounded signal. The signal VP carried on the wire 18 is powered from local power of the active devices on the IEEE 1394 serial bus. Accordingly, at least one of the active devices must be powered by local power. Together, the signals VG and VP form a power signal which is used by the nodes.
A maximum cable length of 4.5 meters is specified for an IEEE 1394 cable. The cabling limitations of an IEEE 1394 serial bus are set by the timing requirements and signal waveform characteristics for transmitted signals. The default timing is set after at most two bus resets, and it is adequate for 32 cable hops, each of 4.5 meters, for a total of 144 meters. This maximum cable length is not practical in some environments in which the distance between active devices is greater than 4.5 meters. One such environment is within an aircraft which can require cable lengths well over 4.5 meters.
U.S. patent application Ser. No. 08/714,659, entitled "IEEE 1394 ACTIVE WALL DISCONNECT AND AIRCRAFT QUALIFIED CABLE" and filed on Sep. 16, 1996, which is hereby incorporated by reference, teaches an IEEE 1394 cable having a length greater than 4.5 meters. The longer cable lengths taught in this application incorporate heavier gauge wire for the two twisted data pairs 12 and 14 in order to match the performance characteristics of a standard IEEE 1394 cable and comply with the signal levels and timing requirements of the IEEE 1394 specification over the increased distance. U.S. patent application Ser. No. 08/714,659 teaches an IEEE 1394 cable having a length of 20 meters including twisted pairs of wire of 18 gauge wire and an IEEE 1394 cable having a length of 30 meters including twisted pairs of wire of 16 gauge wire. While the cables taught in U.S. patent application Ser. No. 08/714,659 achieve longer cable lengths than 4.5 meters and still perform according to the appropriate parameters set by the IEEE 1394 specification, the cables are large in diameter, due to the heavier gauge wire used to achieve the longer length and the thick dielectric material required to maintain signal characteristics. When wiring within a closed environment such as an aircraft where space taken up by the cable is a consideration, large diameter cables are disadvantageous and present problems in assembling and routing the cables. The large diameter cables also add extra weight to the aircraft.
What is needed is a cable for use between IEEE 1394 devices which has a length greater than 4.5 meters and a relatively small diameter and minimum weight.